Method for pattern generation with improved image quality

ABSTRACT

The present invention relates to a method for creating a microlithographic pattern on a workpiece, for increased resolution and image fidelity. The method comprises the steps of: providing a source for emitting electromagnetic radiation, illuminating by said radiation a spatial light modulator (SLM) having several pixels, projecting an image of the modulator on the workpiece, further coordinating the movement of the workpiece, the feeding of the signals to the modulator and the intensity of the radiation, so that said pattern is stitched together from the partial images created by the sequence of partial patterns, where an area of the pattern is exposed at least twice with a change in at least one, and preferably at least two, of the following parameters between the exposures: data driven to the SLM, focus, angular distribution of the illumination at the SLM, pupil filtering, polarisation.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SE99/00296 which has an Internationalfiling date of Mar. 2, 1999, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to printing of patterns with extremelyhigh precision on photosensitive surfaces, such as photomasks forsemiconductor devices and displays. It also relates to direct writing ofsemiconductor device patterns, display panels, integrated opticaldevices and electronic interconnect structures. Furthermore, it can haveapplications to other types of precision printing such as securityprinting. The term printing should be understood in a broad sense,meaning exposure of photoresist and photographic emulsion, but also theaction of light on other light sensitive media such as dry-processpaper, by ablation or chemical processes activated by light or heat.Light is not limited to mean visible light, but a wide range ofwavelengths from infrared (IR) to extreme UV. Of special importance isthe ultraviolet range from 370 nm (UV) through deep ultraviolet (DUV),vacuum ultraviolet (WV) and extreme ultraviolet (EUV) down to a fewmanometers wavelength. EUV is in this application defined as the rangefrom 100 nm and down as far as the radiation is possible to treat aslight. A typical wavelength for EUV is 13 nm. IR is defined as 780 nm upto about 20 μm.

In a different sense the present invention relates to the art andscience of spatial light modulators and projection displays and printersusing such modulators. In particular it improves the gray-scaleproperties, the image stability through focus and image uniformity andthe data processing for such modulators by application of analogmodulation technique. The most important use of the analog modulation isto generate an image in a high-contrast material such as photoresistwith an address grid, i.e., the increment by which the position of anedge in the pattern is specified, that is much finer than the gridcreated by the pixels of the spatial light modulator.

2. Description of Background Art

Precision pattern generators using projection of micromirror spatiallight modulators (SLMs) of the micromirror type are known in the art(Nelson, U.S. Pat. No. 5,148,157, 1988; Kück, EP 0 610 183, 1990). Theuse of an SLM in a pattern generator has a number of advantages comparedto the more wide-spread method of using scanning laser spots. Since anSLM is a massively parallel device, and the number of pixels that can bewritten per second is extremely high. The SLM optical system is alsosimpler in the sense that the illumination of the SLM is non-critical,while in a laser scanner the entire beam path has to be built with highprecision. Compared to some types of scanners, in particularelectrooptic and acoustooptic ones, the micromirror SLM can be used atshorter wavelengths since it is a purely reflective device.

In both references cited above the spatial light modulator uses onlyon-off modulation at each pixel. The input data is converted to a pixelmap with one bit depth, i.e., with the values 0 and 1 in each pixel. Theconversion can be done effectively using graphic processors or customlogic with area fill instructions.

In a previous application by the present inventor Sandström (Sandströmet al., EP 0 467 076, 1990), the ability to use an intermediate exposurevalue at the boundary of a pattern element to fine-adjust the positionof the element's edge in the image created by a laser scanner wasdescribed.

Creation of a gray-scale image is also known in the art, preferably forprojection display of video images and for printing. The is done with anSLM by variation of the time a pixel is turned on or by printing thesame pixel several times with the pixel turned on a varying number oftimes. The present invention devices a system for direct gray-scalegeneration with a spatial light modulator, with a special view to thegeneration of ultra-precision patterns. Important aspects in thepreferred embodiments, are uniformity of the image from pixel to pixeland independence of exact placement of a feature relative to the pixelsof the SLM and stability when focus is changed, either with intention orinadvertently.

Specifically there is a problem with the prior art pattern generation toprovide adequate image resolution and fidelity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an methodto provide an increased resolution and fidelity.

This object is achieved with the method according to the appendedclaims.

The method according to the present invention comprises the steps of

providing a source for emitting electromagnetic radiation in thewavelength range from EW to IR,

illuminating by said radiation a spatial light modulator (SLM) havingmultitude of modulating elements (pixels),

projecting an image of the modulator on the workpiece,

moving said workpiece and/or projection system relative to each other,

further reading from an information storage device a digital descriptionof the pattern to be written,

extracting from the pattern description a sequence of partial patterns,

converting said partial patterns to modulator signals, and feeding saidsignals to the modulator,

further coordinating the movement of the workpiece, the feeding of thesignals to the modulator and the intensity of the radiation, so thatsaid pattern is stitched together from the partial images created by the

sequence of partial patterns,

where an area of the pattern is exposed at least twice with a change inat least one, and preferably at least two, of the following parametersbetween the exposures:

data driven to the SLM

focus

angular distribution of the illumination at the SLM

pupil filtering

polarization.

By using several different exposures, the exposures could be adapted forcertain features and aspects in the pattern, and hereby an improvedresulting pattern is obtained. Thus, the method according to the presentinvention concentrates in a stepwise manner on one difficulty in thepattern at a time.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 shows a printer in prior art. The SLM consists of micromirrorsthat deflect the light from the lens pupil;

FIG. 2 shows a number of pixel designs with the upper four pixels in anoff state and the remaining five pixels turned on;

FIG. 3 shows an array of pixels moving up and down like pistons, therebycreating a phase difference. This is how an edge can be fine-positionedwith a phase-type SLM;

FIGS. 4a, 4 b, 4 c, 4 d, 4 e, 4 f, and 4 g show a schematic comparisonbetween an SLM with deflecting mirrors and an SLM with deformingmirrors;

FIG. 5 shows a flow diagram of a method for translating and feeding datato the SLM;

FIG. 6 shows a preferred embodiment of a pattern generator according tothe invention;

FIGS. 7a and 7 b schematically show how the method according to theinvention could be used for image enhancement by writing different typesof features in the pattern in different exposures;

FIG. 8 shows an example of preferred pupil filter to be used in thepresent invention;

FIG. 9 shows a first schematic example of how a correction exposurecould be used for increasing the resolution of the first exposure; and

FIG. 10 shows a second schematic example of how a correction exposurecould be used for increasing the resolution of the first exposure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basis for understanding the invention is the prior art genericarrangement in FIG. 1 which shows a generic projection printer with anSLM. Spatial light modulators based on reflection come in two varieties,the deflection type (Nelson) and the phase modulation type (Kück). Thedifference between them may in a particular case with micromirrors seemsmall. However, as shown in FIG. 1, the phase SLM extinguishes the beamin the specular direction by destructive interference, while with adeflection SLM, a pixel deflects the specular beam geometrically to oneside so that it misses the aperture of the imaging lens as shown. Forultraprecise printing as performed in the current invention, thephase-modulating system as described by Kück 1990 is superior to thedeflecting type. First, a phase modulation system has better contrastsince all parts of the surface, including the hinges and support posts,take part in the destructive interference and total extinction can beachieved. By contrast, a deflection system works by deflecting the lightto the side and is difficult to make symmetrical around the optical axisat intermediate deflection angles, creating a risk of featureinstability when focus is changed. In the preferred embodiments, thephase modulation type is used. If, however, one accepts or designsaround the asymmetry of the deflection type, a deflection system couldalso be used. This is illustrated schematically in FIG. 4. In FIG. 4a, anon-deflected micromirror 401 is illuminated, and the reflected light isnot directed towards the aperture 402, and, hence, the light does notreach the substrate 403. In FIG. 4b, on the other hand, the mirror isfully deflected, and all the reflected light is directed towards theapperture. In an intermediate position only part of the reflected lightwill reach the substrate, which is shown in FIG. 4c. However, in thiscase the light is not symmetrical around the optical axis for the lens404, and there is an oblique incidence on the substrate. A way to solvethis problem is indicated by FIGS. 4d-f. Here, a first exposure is madewith a first deflection angle for the micromirror, and thereafter asecond exposure, preferably with the same light dose, is made for asecond deflection angle, being complementary to the first angle. Hereby,the combination of the first and second exposure is symmetrical aroundthe optical axis for the lens. Another way to solve the problem is touse deforming mirror 401′, such as is shown in FIG. 4g, whereby thereflected light is evenly distributed over aperture. FIG. 4gschematically represents two cases, a phase type SLM (described below)or a deflection SLM, where light is reflected from different parts ofthe mirror.

The phase SLM can be built either with micromachined-mirrors, so calledmicromirrors, or with a continuous mirror surface on a supportingsubstrate that is deformed using an electronic signal. In Kück 1990, aviscoelastic layer controlled by an electrostatic field is used.However, but it is equally possible, especially for very shortwavelengths where deformations of the order of a few nanometer aresufficient, to use a piezoelectric solid disk that is deformed byelectric field or another electrically, magnetically or thermallycontrolled reflecting surface. For the remainder of this application anelectrostatically controlled micromirror matrix (one- ortwo-dimensional) is assumed, although other arrangements as describedabove are possible, such as transmissive or reflecting SLMs relying onLCD crystals or electrooptical materials as their modulation mechanism,or micromechanical SLMs using piezoelectric or electrostrictiveactuation.

The present invention preferably uses a micromirror where the phasemodulation is variable in order to obtain a variable amount of lightreaching the pupil of the projection lens. FIG. 2 shows eight differentmulti-element. mirrors. The tilts of the various parts of the mirrorsare unimportant. In fact, one element by itself would direct the lighttoward the lens while another would direct it outside of the pupil. Thecorrect way to understand the function is to look at the complexamplitude reaching the center of the pupil from each infinitesimal areaelement of the mirror, and integrate the amplitude over the mirror. Witha suitable shape of the mirror, it is possible to find a deformationwhere the complex amplitudes add up to almost zero, in which case nolight reaching the pupil. This is the off-state of the micromirror. Onthe other hand, a relaxed state, where the mirror surface is flat andthe complex amplitudes add in phase, is the on-state. Between the on andoff-states, the amount of light in the specular direction is acontinuous but non-linear function of the deformation.

The pattern to be written is normally a binary pattern, such as aphotomask pattern in chrome on a glass substrate. In this context,binary means that there are no intermediate areas: a certain point onthe photomask surface is either dark (covered with chrome) or clear (nochrome). The pattern is exposed in photoresist by the projected imagefrom the SLM and the photoresist is developed. Modern resists have highcontrast, meaning that a small percentage change in exposure makes thedifference between full removal of the resist in the developer andhardly any removal at all. Therefore, the photoresist has an edge thatis normally almost perpendicular to the substrate surface, even thoughthe aerial image has a gradual transition between light and dark. Thechrome etching further increases the contrast, so that the resultingimage is perfectly binary: either opaque or clear, with no intermediateareas.

The input data is in a digital format describing the geometry of thepattern to be written on the surface. The input data is often given in avery small address units, e.g.: 1 nanometer, while setting pixels in theSLM to either on or off gives a much coarser pattern. If a pixel on theSLM is projected to a 0.1 μm pixel in the image, a line can only have awidth that is an integer number of pixels (n*0.1 μm, where n is aninteger). An address grid of 0.1 μm was enough until recently, but theadvent of so-called optical proximity correction (OPC) makes a grid of1-5 nanometers desirable. In OPC, the size of features in the mask aremodified slightly to compensate for predicted optical image errors whenthe mask is used. As an example, when a mask with four parallel lines0.8 μm wide is printed in a modern 4× reduction stepper (a projectionprinter for semiconductor wafers), they will typically print as lines0.187, 0.200, 0.200, and 0.187 μm wide, although they were intended tohave the same width. This can be predicted by simulating imageformation, and the user of the mask may use OPC to compensate in themask. Therefore, the user wants the first and last lines in the mask tobe 4*0.213=0.852 μinstead of 0.800 μm. With an address grid of 0.1 μm,the user cannot make the correction, but with a 5 nm address grid orfiner, such correction is possible.

In FIG. 5, the method for providing data for the SLM is shown in a flowdiagram. The first step, S1, is to divide the pattern data for thepattern to be written into separate pattern fields. The pattern data ispreferably received in digital form. Thereafter, in step S2, the fieldsare rasterized, and thereby assigned different exposure values. Thesevalues are then corrected for nonlinear response, step S3, andpixel-to-pixel variations, step S4. Finally, the pixel values areconverted to drive signals and forwarded to the SLM.

The invention preferably uses intermediate values between the off-stateand on-state to create a fine address grid, e.g., 1/15, 1/25, 1/50, ofthe size of a pixel. A printed feature consists of pixels in theon-state, but along the edge, it has pixels set to intermediate values.This is done by driving the pixels with voltages other than the on- andoff voltages. Since there are several cascaded non-linear effects (forinstance, the position of the edge versus exposure at the pixels at theboundary, the exposure vs. the deformation, and the deformation vs. theelectric field), a non-linear transformation from the input data to theelectric field is needed. Furthermore, this transformation is calibratedempirically at regular time intervals.

FIG. 3 shows an array of pixels moving up and down like pistons, therebycreating a phase difference. The figure shows how the pixels arecontrolled to create the reflectivity in the inset. The bright areashave pixels with 0 phase, while dark areas are created by pixels withalternating +90 and −90 degree phases. The oblique boundaries betweenbright and dark areas are created by intermediate values of phase. Thisis how an edge can be fine-positioned with a phase-type SLM. However,other types of SLM with intermediate values could be used in the samemanner. The imaging properties with the phase SLM driven intointermediate values are complex, and it is far from obvious that theedge will be moved in FIG. 3. However, it has been shown by extensivetheoretical calculations and experiments by the inventor that thedescribed effect is real.

The Design of the Phase-type SLM

A cloverleaf mirror design as used in prior art makes it possible todrive to intermediate states between on and off states. However, whenthe integrated complex amplitude is plotted as a function of deflection,it is seen that it never goes to zero completely, but hovers aroundzero, therefore having a non-zero minimum reflectivity with a varyingphase angle. A thorough analysis of an image with some pixels set tointermediate states shows that the position of the edges in the finalimage is not stable through focus if the integrated phase angle of theedge pixels is not zero. In a preferred embodiment of the presentinvention, a new type of pixel with pivoting elements is used. When theelements pivot, one end moves toward the light source, and the other endaway from it, the average phase keeping close to zero. Furthermore, thecloverleaf design has a problem of built-in stress created duringfabrication. This stress tends to give a partial deformation without anapplied electric field. The built-in deformation is not perfectly thesame in every pixel, since it depends on imperfections duringmanufacturing. In the cloverleaf design, this difference from pixel topixel creates a first-order. variation of reflectivity. With pixel cellsbuilt from pivoting elements, the same effect occurs, but gives asecond-order effect. Therefore, the uniformity of the projected image isbetter.

Image Enhancements

There is a third advantage with a pivoting design. Specifically, thecloverleaf does not reach full extinction, a pivoting cell can moreeasily be given a geometry that gives full extinction, or even goesthrough zero and comes back to a small non-zero reflection, but withreversed phase. With better extinction there is greater freedom to printoverlapping exposures, and designing for a small negative value givesbetter linearity close to extinction. Printing with a weak exposure,approximately 5%, in the dark areas, but with reversed phase can give anincreased edge sharpness of 15-30% and the ability to print smallerfeatures with a given lens. This is an analog to so-called attenuatingphase-shifting masks that are used in the semiconductor industry. Arelated method of increasing the edge acuity is to set the pixels thatare inside a feature a lower value and those near the edge a highervalue. This gives a new type of image enhancement not possible withcurrent projection of patterns from masks or by the use of projectorsfollowing Nelson and Kück. The combination of a non-zero negativeamplitude in the background and an increased exposure along the edgesneed not conflict with the creation of a fine address grid by drivingedge pixels to intermediate values, since the effects are additive or atleast computable. When the pixels are substantially smaller than thefeature to be printed a combination of pixel values exists, creating alleffects simultaneously. To find them requires more computation than thegeneration of a fine address grid alone, but in some applications of thepresent invention the ability to print smaller features can have a highvalue that pays for the extra effort.

In the case of a continuous mirror on a viscoelastic layer, there is aninherent balancing of the average phase to zero. Simulations have shownthat driving to intermediate values for fine positioning of featureedges also works for the continuous mirror. The non-linearities aresmaller than with micromirrors. However, for the method to work well theminimum feature has to be larger than with micromirrors, i.e., have alarger number of addressed pixels per resolved feature element isneeded. The causes the neeed for a larger SLM device, and for givenpattern, the amount of data is larger. Therefore, the micromirrors havebeen chosen in a first and second embodiment.

In present the invention a pixel with rotation-symmetrical deformation(at least two-fold symmetry, in a preferred embodiment four-foldsymmetry) is used for two reasons: first, to give a symmetricalillumination of the pupil of the projection lens, and second, to makethe image insensitive to rotations. The latter is important for printinga random logic pattern on a semiconductor wafer. If there is an xyasymmetry, the transistors laid-out along the x axis will have adifferent delay from those along the y axis, and the circuit maymalfunction or can only be used at a lower clock-speed. The tworequirements of image consistency through focus and symmetry between xand y makes it very important to create and maintain symmetries in theoptical system. Symmetry can be either inherent or it can be created bydeliberate balancing of asymmetric properties, such as using multipleexposures with complementary asymmetric properties. However, sincemultiple exposures lead to reduced through-put, inherent symmetricallayouts are strongly favored.

The present invention preferably uses several, or at least two,exposures for at least some area of the pattern, whereby a change in atleast: one, and preferably at least two, of the following parameters aremade between the exposures:

data driven to the SLM

focus

angular distribution of the illumination at the SLM

pupil filtering

polarization.

For example, one exposure could have a higher exposure dose and create apattern at the workpiece, and the second exposure have a lower exposuredose, an essentially inverse modulation at the SLM and a pupil filter,reducing the optical resolution at the workpiece. Alternatively, theexposures could have essentially the same exposure dose and adirectional pupil filter (e.g., elliptical, rectangular or wedgeshaped), could be rotated between the exposures. Also, the pattern datadriven to the SLM could have a directional asymmetry, the asymmetrybeing rotated between the exposures, or further, the polarization of thelight impinging on the workpiece could be rotated between the exposures.

If at least two of the following parameters are changed between theexposures:

data driven to the SLM

angular distribution of the illumination at the SLM

pupil filtering

polarization,

their directional properties could be rotated between the exposures, therotation being synchronized between the at least two parameters.

In FIGS. 7a and 7 b, shown schematically are how two exposures withdifferent (rotated) properties could be used to acheive a pattern withincreased resolution. Here, the first exposure is directed to writingfeatures in one direction, whereas the second exposure is directed towriting features in an perpendicular direction. In FIG. 7a, data forpattern features in a first direction are sent to the modulator 701, andthe filter is rotated to a direction most suitable for these patternfeatures. Thereafter pattern features in the second direction is drivento the modulator, and the filter is rotated to a perpendicular direction(FIG. 7b).

Preferably, the pupil filter used according to the present invention hasa dogbone shape for the writing of patterns with narrow parallel) lines.Such a filter is shown in FIG. 8a. Further, the pupil filter could havean area for attenuating the spatial component of the light correspondingto large features in pattern, shown in FIG. 8b. Such a filter ispreferably used for oblique illumination. The pupil filter could also bea secondary spatial light modulator. This secondary SLM, or a tertiarySLM, could also be used to control the angular distribution of theillumination at the primary SLM, creating a fully software drivenimplementation of the present invention. Finally, FIG. 8c shows anexample of a filter with a phase shifting inner circle, used forimproved resolution.

FIG. 9 shows how the data used in the second exposure could be adaptedto increase the image resolution. In the first exposure, the curve 901is written. In this case, a significant deviation between the intendedpattern to be written, indicated by the dashed line 902, and the curve901 actually written. The top levels varies, especially for narrowfeatures, making the size of the features difficult to control. In thesecond exposure inverse data are used, as is shown for the curve 903.However, in this case, a smoothening filter is used for the secondexposure, making the curve actually written to be curve 904. Togetherwith the first exposure, the illumination curve will now look like thecurve 905, where the top levels are essentially the same for all theprinted features. By using a higher processing limit for the developmentprocess, indicated by the dashed lines 906 and 906′, the resolution inthe pattern is hereby significantly increased. However, the contrast isat the same time lowered, and there must be a trade-off between thecontrast and the resolution. For patterns with very narrow features,indicated by the dotted line 1002 in FIG. 10, the difference between thecurve 1001 written in the first exposure and the intended pattern areeven more significant. In this case an inverse pattern 1003 with lesssmoothening is written in the second exposure, indicated by 1004. Herebythe resolution is improved even for very small features, but again atthe cost of a deteriorated contrast, as indicated by the processinglimits 1006 and 1006′, respectively.

Preferred Embodiments

A first preferred embodiment is a deep-UV pattern generator forphotomasks using an SLM of 2048×512 micromirrors. The light source is anKrF excimer laser with a pulsed output at 248 nanometers, pulse lengthsof approximately 10 ns and a repetition rate of 500 Hz. The SLM has analuminum surface that reflects more than 90% of the light. The SLM isilluminated by the laser through a beam-scrambling illuminator and thereflected light is directed to the projection lens and further to thephotosensitive surface. The incident beam from the illuminator and theexiting beam to the lens are separated by a semitransparentbeam-splitter mirror. Preferably the mirror is polarization-selectiveand the illuminator uses polarized light, the polarization direction ofwhich is switched by a quarter-wave plate in front of the SLM. For x andy symmetry at high NA the image must be symmetrically polarized and asecond quarter-wave plate between the beam-splitter and the projectionlens creates a circularly polarized image. A simpler arrangement whenthe laser pulse energy allows it is to use a non-polarizing beamsplitter. The quarter-wave plate after the second pass through the beamsplitter is still advantageous, since it makes the design of thebeam-splitting coating less sensitive. The simplest arrangement of allis to use an oblique incidence at the SLM so that the beams from theilluminator and to the projection lens are geometrically separated, asin FIG. 1.

The micromirror pixels are 20×20 μm and the projection lens has areduction of 200X, making on pixel on the SLM correspond to 0.1 μm inthe image. The lens is a monochromatic DW lens with an NA of 0.8, givinga point spread function of 0.17 μm FWHM. The minimum lines that can bewritten with good quality are 0.25 μm.

The workpiece, e.g., a photomask, is moved with aninterferometer-controlled stage under the lens and the interferometerlogic signals to the laser to produce a flash. Since the flash durationis only 10 ns the movement of the stage is frozen during the exposureand an image of the SLM is printed, 204.8×51.2 μm large. Twomilliseconds later the stage has moved 51.2 μm, a new flash is shot, anda new image of the SLM is printed edge to edge with the first one.Between the exposures the data input system has loaded a new image intothe SLM, so that a larger pattern is composed of the stitched flashes.When a full column has been written, the stage advances in theperpendicular direction and a new row is started. Any size of patterncan be written in way, although the first preferred embodiment typicallywrites patterns that are 125×125 mm. To write this size of pattern takes50 minutes, plus the time for movement between consecutive columns.

Each pixel can be controlled to 25 levels (plus zero), therebyinterpolating the pixel of 0.1 μm into 25 increments of 4 nanometerseach. The data conversion takes the geometrical specification of thepattern and translates it to a map with pixels set to on, off, or to anintermediate reflection. The data path must supply the SLM with2048*512*500 words of data per second, in practice 524 Mbytes of pixeldata per second. In a preferred embodiment, the writable area ismaximally 230×230 mm, giving up to 230/0.0512=4500 flashes maximum in acolumn and the column is written in 4500/500=9 seconds. The amount ofpixel data needed in one column is 9×524=4800 Mb. To reduce the amountof transferred and buffered data, a compressed format is used, similarto the one in Sandström at al. 1990, but with the difference that apixel map is compressed instead of segments with a length and a value. Aviable alternative is to create a pixel map immediately and usecommercially available hardware processors for compression anddecompression to reduce the amount of data to be transferred andbuffered.

Even with compression, the amount of data in a full mask makes it highlyimpractical to store pre-fractured data on disk, but the pixel data hasto be produced when it is used. An array of processors rasterizes theimage in parallel into the compressed format and transfers thecompressed data to an expander circuit feeding the SLM with pixel data.In a first preferred embodiment, the processors rasterize differentparts of the image and buffer the result before transmitting it to theinput buffer of the expander circuit.

A Second Preferred Embodiment

In a second preferred embodiment, the laser is an ArF excimer laser with193 nm wavelength and 500 Hz pulse frequency. The SLM has 3072×1024pixels of 20*20 μm, and the lens has a reduction of 333X, giving aprojected pixel of 0.06 μm. There are 60 intermediate values, and theaddress grid is 1 nanometer. The point spread function is 0.13 μm, andthe minimum line 0.2 μm. The data flow is 1572 Mbytes/s, and the data inone column 230 mm long is 11.8 Gb.

A Third Embodiment

A third preferred embodiment is identical to the second embodiment,except that the matrix of pixels is rotated 45 degrees and the pixelgrid is 84 μm, giving a projected pixel spacing along x and y of 0.06μm. The laser is an ArF excimer laser, and the lens has a reduction of240. Because of the rotated matrix, the density of pixels in the matrixis less, and the data volume is half of the previous embodiment.Nonetheless, the address resolution is the same.

Laser Flash-to-flash Variations

The excimer laser has two unwanted properties, flash-to-flash energyvariations of 5% and flash-to-flash time jitter of 100 ns. In thepreferred embodiments, both are compensated for in the same way. A firstexposure is made of the entire pattern with 90% power. The actual flashenergy and time position for each flash are recorded. A second exposureis made with nominally 10% exposure, with the analog modulation used tomake the second exposure 5-15%, depending on the actual value of thefirst exposure. Likewise, a deliberate time offset in the secondexposure can compensate for the time jitter of the first exposure. Thesecond exposure can fully compensate for the errors in the firstexposure, but will itself result in new errors of the same type. Sinceit is only on average 10% of the total exposure, both errors areeffectively reduced by a factor of ten. In practice, the laser has atime uncertainty that is much larger than 100 ns, since the light pulsecomes after a delay from the trigger pulse, and this delay varies by acouple of microseconds from one time to another. Within a short timespan, the delay is more stable. Therefore, the delay is measuredcontinuously, and the last delay values, suitably filtered, are used topredict the next pulse delay and to position the trigger pulse.

It is possible to make corrections for stage imperfections in the sameway, namely, if the stage errors are recorded and the stage is drivenwith a compensating movement in the second exposure. Any placementerrors that can be measured can, in principle, be partially or fullycorrected in this way. It is necessary to have a fast servo to drive thestage to the computed points during the second exposure. In prior art,the SLM is self-mounted on a stage with small stroke and short responsetime and used for fine positioning of the image. Another equally usefulscheme is to use a mirror with piezoelectric control in the opticalsystem between the SLM and the image surface, the choice between the twobeing made based on practical considerations. It is also possible to adda position offset to the data in an exposure field, to thereby move theimage laterally.

The second exposure is preferably taken with an attenuating filterbetween the laser and the SLM so that the full dynamic range of the SLMcan be used within the range 0-15% of the nominal exposure. With 25intermediate levels, it is possible to adjust the exposure in steps of15%*1/25=0.6%.

The response varies slightly from pixel to pixel due to manufacturingimperfections, and possibly from aging also. The result is an unwantedinhomogeneity in the image. Where image requirements are very high, itmay be necessary to correct every pixel by multiplication, wherein thepixels inverse responsiveness is stored in a lookup memory. Even betteris the application of a polynomial with two, three, or more terms foreach pixel. This can be done in the logic of the hardware that drivesthe SLM.

In a more complex preferred embodiment, several corrections are combinedinto the second corrective exposure: the flash-to-flash variation, flashtime jitter, and also the known differences in the response between thepixels. As long as the corrections are small, i.e., a few percent ineach they will add approximately linearly, therefore the corrections canbe simply added before they are applied to the SLM. The sum ismultiplied with the value desired exposure dose in that pixel.

Alternative Illumination Sources

The excimer laser has a limited pulse repetition frequency (prf) of500-1000 Hz, depending on the wavelength and type of the laser. Thisgives large fields with stitching edges in both x and y axes. In twoother preferred embodiments, the SLM is illuminated with a pulsed laserwith much higher prf, e.g., a Q-switched, upconverted solid state laser,and with a continuous laser source scanned over the surface. of the SLM,so that one part of the SLM is reloaded with new data while another partis printed. In both cases the coherence properties of the lasers aredifferent from the excimer laser and a more extensive beam-scramblingand coherence control is needed, e.g., multiple parallel light pathswith different path lengths. In some implementations of the inventionthe light output from a flash lamp is sufficient and can be used as thelight source. Advantages are low cost and good coherence properties.

In the preferred embodiment with scanning illumination two issues areresolved. First, the pulse-to-pulse variation in time and energy isimproved, since the scanning is done under full control preferably usingan electrooptic scanner such as acoustooptic or electrooptic, andsecond, many continuous laser have less power fluctuation than pulsedlasers. Furthermore the use of continuous lasers gives a differentselection of wavelengths. Also, continuous lasers are less dangerous tothe eye than pulsed lasers. Most important, however, is the possibilityof reaching much higher data rates with a matrix with only a few lines,since the scanning is non-critical and can be done with a 100 kHzrepetition rate or higher. Scanning the illumination beam is also a wayof creating a very uniform illumination, which is otherwise difficult.

In some embodiments it is possible, and feasible, to use a flash lamp asthe illumination source.

Extreme Ultraviolet Devices

Light sources for extreme ultraviolet (EUV) are based on radiation froma particle accelerator, a magnetic plasma pinch machine, or by theheating of a small drop of matter to extreme temperatures with ahigh-power laser pulse. In either case the radiation is pulsed. The EUVradiation propagates only in vacuum and can only be focused byreflective optics. A typical pattern generator using an SLM has a smallexposure field and a modest requirement of optical power. The design ofthe optical system is therefore relaxed compared to that of an EWstepper, making it possible to use more mirrors and go to higher NA thanin a stepper. It is anticipated that a high-NA lens will have aring-shaped exposure field, and it is fully possible to adapt the shapeof the SLM to such a field. With a wavelength of 13 nm and an NA of 0.25it is possible to expose lines that are only 25 nm wide, and, usingimage enhancement as described below, even below 20 nm. No other knownwriting technology can match this resolution and at the same timeachieve the writing speed that is made possible by the parallelcharacter of an SLM.

Edge Overlap

Since a two-dimensional field is printed for each flash and the fieldsare stitched together edge to edge to edge, the stitching is verycritical. A displacement of only a few nanometers of one field willcreate pattern errors along that edge that are visible and potentiallydetrimental to the function of an electronic circuit produced from themask. An effective way of reducing the unwanted stitching effects is toprint the same pattern in several passes, but with a displacement of thestitching boundaries between the passes. If the pattern is printed fourtimes the stitching error will occur in four positions, but with onlyone-fourth of the magnitude. In a preferred embodiment of the presentinvention, the ability to create intermediate exposures is used togetherwith an overlap band between the fields. The values are computed duringthe rasterization, although it could also be done during the expansionof the compressed data. Edge overlap reduces the stitching errors withmuch less throughput penalty than multipass printing.

Modified Illumination

In the first preferred embodiment, the illumination of the SLM is doneby an excimer laser and a light scrambler such as a fly-eye lens arrayto create an illumination that resembles that from a circularself-luminous surface in the pupil plane of the illuminator. In order toincrease the resolution, when printing with a specific projectionsystem, it is possible to use a modified illumination. In the mostsimple cases, pupil filters are introduced in the pupil plane of theilluminator, e.g., with a quadrupole-shaped or annular transmissionarea. In a more complex case the same field is printed several times.Several parameters can be made to vary between the exposures, such asfocus in the image plane, illumination pattern, data applied to the SLM,and pupil filter in the pupil plane of the projection optics. Inparticular the synchronized variation of the illumination and a pupilfilter can give an increased resolution, most notably if the pupil hasand a sector-shaped transmitting area, and the illumination is alignedso that the non-diffracted light intercepts an absorbing stop near theapex of the sector.

Linearization of the Response

There are essentially three approaches for linearization of the transferfunction from data to edge placement:

taking the non-linearity into account in the data conversion unit, andgenerating an 8 bit (example) pixel values. in the data conversion unit,and using digital-to-analog converters (DACs) with the same resolutionto drive the SLM.

to generate digital values with fewer values, e.g., 5 bits or up to 32values, and translate them to an 8 bit value in a look-up table (LUT),and then feed the 8 bit values to the DACs.

to use a 5 bit value and semiconductor switches to select a DC voltagethat is generated by one or several high-resolution DACs.

In either case, it is possible to measure an empirical calibrationfunction such that the response on the plate is linearized, when saidfunction being applied at respectively the data conversion unit, the LUTor in the DC voltages.

Which linearization scheme to use depends on the data rate, theprecision requirements and also on the available circuit technology thatmay change over time. At the present time the data conversion unit is abottleneck and therefore it is not a preferred solution to linearize inthe data conversion unit, neither to generate 8-bit pixel values.High-speed DACs are expensive and power-hungry. Thus, the mostappropriate solution is to generate DC voltages and use switches. It isthen possible to use even higher resolution than 8 bits.

Description of a Preferred Pattern Generator

Referring to FIG. 6, a pattern generator comprises an SLM 601 withindividual and multi-value pixel addressing, an illumination source 602,an illumination beam scrambling device 603, an imaging optical system604, a fine positioning substrate stage 605 with an interferometerposition control system 606 and a hardware and software data handlingsystem 607 for the SLM. For proper functionality and ease of operationit also contains a surrounding climate chamber with temperature control,a substrate loading system, software for timing of stage movement andexposure laser triggering to achieve optimum pattern placement accuracy,and a software user interface.

The illumination in the pattern generator is done with a KrF excimerlaser giving a 10-20 nanoseconds long light flash in the UV region at248 nanometer wavelength with a bandwidth corresponding to the naturalline width of an excimer laser. In order to avoid pattern distortion onthe substrate, the light from the excimer laser is uniformly distributedover the SLM surface and the light has a short enough coherence lengthnot to produce laser speckle on the substrate. A beam scrambler is usedto achieve two objectives. First, the beam scrambler divides the beamfrom the excimer laser in several beam paths with different path length,and second, it adds them together in order to reduce the spatialcoherence length. The beam scrambler also has a beam homogenizesconsisting of a lens system containing a set of fly-eye lenses thatdistributes the light from each point in the laser beam from the excimerlaser uniformly over the SLM surface giving a “top-hat” lightdistribution.

The light from the SLM is relayed and imaged down to the substrate onthe substrate stage. This is done using a Schlieren optical systemdescribed by Kück. A lens 11 with the focal width f_(l) is placed at thedistance fl from the SLM. Another lens 12 with the focal length fz isplaced at the distance 2×f_(l)+f₂ from the SLM. The substrate is then ata distance 2×f_(l)+2×fz from the SLM. At the distance 2×f₁ from the SLMthere is an aperture 608 which size determines the numerical aperture(NA) of the system and thereby the minimum pattern feature size that canbe written on the substrate. In order to correct for imperfections inthe optical system and the substrate flatness, there is also a focalsystem that dynamically positions the lens 12 in the z direction with aposition span of 50 micrometers to achieve optimal focal properties. Thelens system is also wavelength corrected for the illuminating wavelengthof 248 nanometers and has a bandwidth tolerance of the illuminatinglight of at least ±1 nanometer. The illuminating light is reflected intothe imaging optical system using a beam splitter 609 that is positionedimmediately above the lens 11. For a demagnification factor of 250 andan NA of 0.62 it is possible to expose pattern features with a size downto 0.2 micrometers with a good pattern quality. With 32 gray levels fromeach SLM-pixel the minimum grid size is 2 nanometers.

The pattern generator has a fine positioning substrate stage with aninterferometer position control system. It consists of a moveable airbearing xy table 605 made of zerodur for minimum thermal expansion. Aservo system with an interferometer position feedback measuring system606 controls the stage positioning in each direction. In one direction,y, the servo system keeps the stage in fixed position and in the otherdirection, x, the stage moves with continuous speed. The interferometerposition measuring system is used in the x-direction to trigger theexposure laser flashes to give uniform position between each image ofthe SLM on the substrate. When a full row of SLM images are exposed onthe substrate, the stage moves back to the original position in the xdirection, and further moves one SLM image increment in the y directionto expose another row of SLM images on the substrate. This procedure isrepeated until the entire substrate is exposed.

The SLM images overlaps with a number of pixels in both the x and ydirection, and the exposure data pattern is locally modified in theoverlap pixels to compensate for the increased number of exposures thatresult in such overlap areas.

Variations in pulse to pulse intensity from the excimer laser iscompensated by using two-pass exposure. of the pattern, whereby thefirst pass is done using a nominal 90% intensity of the correctintensity. In the first pass, the. actual intensity in each laser flashis measured and stored. In the second pass, the correct intensity foreach SLM image exposure is then used based on the measured intensityvalues from the first pass. In this way, it is possible to reduce theinfluence from pulse-to-pulse intensity variations from the excimerlaser by one order of magnitude.

The functionality of the SLM is described extensively elsewhere in thistext. It has 2048×256 pixels, with a pixel size of 16 micrometers, andit is possible to address all pixels within 1 millisecond. The SLM isrigidly mounted in a fine stage. This fine stage is moveable 100 micronsin the x and y directions, and has an accuracy better than 100nanometers between each flash exposure. The fine positioning of the SLMis used to correct for position inaccuracy of the substrate positioningstage to further reduce pattern-stitching errors. In addition to the x-ypositioning, there is also a rotational possibility of the SLM stage, inorder to expose a pattern on a substrate with an angle other than theone specified by the substrate stage coordinate system. The purpose forsuch rotation is to create the possibility of incorporating substratealignment feasibility for substrates with an already existing patternwhere additional features shall be added. It is possible to measure theexact position of the substrate on the stage after loading it, using anoff-line optical channel and a CCD camera to determine the systemcoordinates for a number of alignment marks existing on the substrate.During exposure, the stage position is then corrected in the x- andy-directions based on the measured positions of the alignment marks.Rotational alignment is achieved by using the stage servo system tofollow the rotated coordinate system, and also rotating the SLM finestage as described above.

An arbitrary data pattern of an arbitrary format is transformed into acompressed rasterized pixel map with 32 (5 bit) gray levels per pixel ina pattern rasterizer 610. Since the gray scale steps of an exposed pixelis not linear in response to the voltage applied to the pixel electrode,the input data is linearized in a pixel linearizer 611, so that the 32gray levels correspond to a uniform increase in exposure dose for eachsuccessive level. This is done using 8-bit digital-to-analog converters(DAC) 612, whereby each gray level from the pixel map selects a voltagefrom the DACs according to a previously empirically calibratedlinearization function. An additional offset in the choice of analoglevel from the DACs is made using a lookup table where each valuecorresponds to an SLM pixel and each such value corrects for anomaliesof the corresponding pixel. The calibration values in the lookup tableare generated using an empirical calibration procedure where a series oftest patterns are sent to the SLM and the resulting exposed patterns aremeasured and used for individual pixel correction. This means that eachgray level in the pixel map selects an analog voltage generating a pixeldeformation for every corresponding SLM pixel to give the correctexposure dose.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art to beincluded within the scope of the following claims.

What is claimed:
 1. A method for creating a microlithographic pattern ona workpiece sensitive to radiation, for increased resolution and imagefidelity, comprising the steps of: providing a source for emittingelectromagnetic radiation in the wavelength range from extremeultraviolet (EUV) to infrared (IR), illuminating by said radiation aspatial light modulator (SLM) having a multitude of modulating elements(pixels), projecting an image of the modulator on the workpiece, movingsaid workpiece and/or projection system relative to each other, furtherreading from an information storage device a digital description of thepattern description a sequence of partial patterns, extracting from thepattern description a sequence of partial patterns, converting saidpartial patterns to modulator signals, and feeding said signals to themodulator, further coordinating the movement of the workpiece, thefeeding of the signals to the modulator and the intensity of theradiation, so that said pattern is stitched together from the partialimages created by the sequence of partial patterns, wherein an area ofthe pattern is exposed at least twice with a change in at least one, andpreferably at least two, of the following parameters between theexposures: data driven to the SLM focus angular distribution of theillumination at the SLM pupil filtering polarization.
 2. A methodaccording to claim 1, where one exposure has a higher exposure dose andcreates a pattern at the workpiece, and the second exposure has a lowerexposure dose, an essentially inverse modulation at the SLM and a pupilfilter reducing the optical resolution at the workpiece.
 3. A methodaccording to claim 1, wherein exposures have essentially the sameexposure dose and wherein a directional pupil filter is rotated betweenthe exposures.
 4. A method according to claim 1, where exposures haveessentially the same exposure dose and where the pattern data driven tothe SLM has a directional asymmetry and said asymmetry is rotatedbetween the exposures.
 5. A method according to claim 1, where exposureshave essentially the same exposure dose and where the polarization ofthe light impinging on the workpiece is rotated between the exposures.6. A method according to claim 1, where exposures have essentially thesame exposure dose and where the focus is changed between the exposures.7. A method according to claim 1, wherein at least two of the followingparameters are changed between the exposures and said two changedparameters having a directional property: data driven to the SLM angulardistribution of the illumination at the SLM pupil filteringpolarization, and said directional property is rotated between theexposures and the rotation is synchronized between the at least twoparameters.
 8. A method according to claim 1, wherein the pupil filterhas a dog bone shape.
 9. A method according to claim 1, where the pupilfilter has an area for attenuating the spatial component of the lightcorresponding to large features in pattern.
 10. A method according toclaim 1, wherein the pupil filter is a secondary spatial lightmodulator.
 11. A method according to claim 1, wherein the angulardistribution of the illumination at the primary SLM is controlled by anillumination controlling SLM.
 12. A method according to claim 3, whereinthe directional pupil filter is elliptical.
 13. A method according toclaim 3, wherein the directional pupil filter is rectangular.
 14. Amethod according to claim 3, wherein the directional pupil filter iswedge-shaped.